RICE UNIVERSITY

STRUCTURAL GEOLOGY OF THE GASS PEAK AREA LAS VEGAS RANGE, NEVADA

by

WILLIAM JAMES EBANKS, JR.

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF ARTS

3 1272 00020 4303

Thesis Director's Signature

Houston, Tex.z (May, 1965) Abstract

A field-mapping study of the area around Gass Peak, Las Vegas Range, Clark County, Nevada, has demonstrated the presence of a thick sequence of Paleozoic and Precambrian

rocks overthrust on younger, Upper Paleozoic beds along the Gass Peak Thrust.

Formations recognized in the upper plate of the thrust are the Precambrian Stirling Quartzite, the Precambrian and Cambrian Wood Canyon Formation, the Cambrian, Carrara,

Bonanza King, and Nopah Formations, and the Ordovician lowermost Pogonip Group. Rocks in the lower plate are the Devonian Sultan Limestone, Mississippian Monte Cristo Lime¬

stone, and the Permo-Carboniferous Bird Spring Formation. The Miocene Horse Spring Formation unconformably overlies the Paleozoic rocks. Faulting on the Gass Peak Thrust resulted in approxi¬ mately 18,000 feet of stratigraphic displacement from west- to-east. Several large folds and many high-angle reverse

faults are associated with the thrust fault. All of these features, the major thrust and smaller re¬ lated structures, have been rotated westward through 90 de¬ grees by right-lateral strike-slip movement on the adjacent

Las Vegas Valley Shear Zone. The "drag" structure and a unique set of faults in the Gass Peak area are related to east-west extension and north-south compression caused by i the rotation. i Strata above and below the Gass Peak Thrust are similar to strata in the Wheeler Pass Thrust, suggesting the two

thrusts are offset equivalents. This implies more than 25 miles of relative horizontal shift between the two faults. Many details of structure are dissimilar between the Wheeler

Pass and Gass Peak Thrusts, and if they were once the same feature, they have developed independently after being separ¬ ated. Structural evidence indicated that displacement on the Gass Peak Thrust occurred before movement along the Las Vegas Valley Shear Zone. There is little evidence in the mapped area for Tertiary block faulting. TABLE OF CONTENTS

page no#

I, INTRODUCTION . 1

A. PURPOSE OF INVESTIGATION 1

B. ACKNOWLEDGEMENTS ...... „. 1

C o PREVIOUS WORK oe..,oo,a.oo,.*«o,ooooa 2

II, LOCATION AND DESCRIPTION OF THE STUDY AREA 3

III, STRATIGRAPHY •,»oo»,,o«o*«,,oaeo, 3

A. METHODS ...... 5

B. GENERAL 5

C. FORMATIONS PRESENT .,... 7

1, Precambrian rocks 7 a. Stirling Quartzite 7 2• Cambrian rocks 8 a. Wood Canyon Formation 8 b. Carrara Formation .. 9 c. Bonanza King Formation •. 0 •.... 10 d. Nopah Formation •. 11 3, Ordovician rocks ...... , 13 a, Pogonip Group 13 4, Devonian-Mississippian rocks (undifferentiated) 14 a, Monte Cristo Limestone and Sultan Limestone ...... 14 5, Mississippian-Pennsylvanian- Permian rocks ..o.. 14 a. Bird Spring Formation 14 6, Tertiary rocks 16 a. Horse Spring Formation ... 16 7, Quaternary rocks •»••«,«.. 19 a. Older and younger alluvium ,,,, 19

IV, STRUCTURAL GEOLOGY ,««o,oo,,20

A, REGIONAL SETTING 20

B. STRUCTURAL FEATURES OF THE GASS PEAK AREA ,e,0o,,,,,ea,e,,,,o,,,a*oeooooo, 21

U Gass Peak Thrust 21 a. General ,., 21 b. Stratigraphic displacement 22 c. Geometry o,*«,23 ii

d. Footwall 23 e. Hanging wall 24 f. Decollement faulting 26 g. Other features 27 2» Structures related to thrusting • • 30 a. General ...... 30 b. Footwall features ... 30 Cl) Broad folds 30 (2) Reverse faults ...a...... 30 (3) Overturned folds 31 (4) Normal faults 33 c. Hanging wall features 37 (1) High-angle reverse faults • 37 (2) Broad folds 38 3. Structures related to the Las Vegas Valley Shear Zone 38 a. General ... 38 b. Extension faults 39 c. Back thrusts 41 4. Normal faulting 43 5. Late Tertiary deformation ...»..*• 44 6. Gravity sliding 44

C. TIMES OF MAJOR DEFORMATIONS 46 1. Thrust faulting 46 2. Strike-slip faulting 47 V. SUMMARY 48

VI. CONCLUSIONS 50

VII. BIBLIOGRAPHY 53

LIST OF ILLUSTRATIONS

Figure 1 Index map of U.S.G.S. Quadrangles in the area of Las Vegas, Nevada . 4 Figure 2 Stratigraphic units of the Gass Peak area, Nevada «...o.• ••«..«..«...... o.. 6 Figure 3 Major structural features near the • Gass Peak area, Nevada 18

Figure 4 Drag structures ...... o«.o*.«o...... 40 Figure 5 Small-scale folding in the Bird Spring Formation ...... e. 25/ r 1 iii

Figure 6 Overturned beds south of Dry Wash ... 32 Figure 7 Overturned beds south of Broken Hill 34 Figure 8 Normal fault dipping southward 35 Figure 9 Low-angle normal faults dipping northward 36

PLATE I Geologic Map of part of the Gass Peak Quadrangle, Clark County, Nevada •••• pocket

- •

I I 1.

INTRODUCTION

PURPOSE OF INVESTIGATION

This study was undertaken to describe the structural geology in the area at the southern end of the Gass Peak Thrust, in the Las Vegas Range, Nevada. A more specific goal was the observation of structural features that might confirm or reject the postulated presence of a major strike-

slip fault zone in the adjacent Las Vegas Valley. Of secondary interest, but significant to the interpretation of the geology of this complex area, was recognition of the stratigraphic units present in the mapped area. ACKNOWLEDGEMENTS

/ This geologist wishes to express his appreciation to the several persons whose help and criticism contributed to the completion of this thesis. First among these is Dr. B. C« Burchfiel, whose guidance in the field and super¬ vision were a valuable experience. Dr. J. Cl. De Bremaecker and Dr. R. R. Lankford also offered constructive criticism in discussions and in editing this thesis. Dr. J. L. Wilson, of Shell Development Company, kindly offered expert opinion on several Cambrian fossil identifications.

The subject area is mostly within the U.S, Fish & Itfild- life Service, Desert Game Range, near Las Vegas, Nevada. The Manager of this refuge granted the writer complete freedom of access to that area. For the kind help and warm hospitality of the good people at the Corn Creek Field Station, the writer I .. is especially grateful. 2

Financial support of the writer was provided by a National Defense Education Act Fellowship. The Rice Uni¬ versity Geology Department provided field expense assistance. PREVIOUS WORK

The expedition led by J. E. Spurr (1903) to explore the geology of Nevada south of the Fortieth Parallel was the first party of geologists to enter and report on the Gass Peak area. Spurr himself passed across the north end of the

Las Vegas Range, but R. B. Rowe, discoverer of the Keystone Thrust, traveled the Mormon Wells Road, through the Gass Peak area, down to Las Vegas Valley. The text and map of Spurr's report indicate a slightly different topographic nomenclature than is presently used. The "Gass" or "Sheep Mountain" of that time is today called Gass Peak. Spurr's usage of "Las

Vegas Range" and "New Mountains" have also been changed.

Nevertheless, from Spurr's writing, it is clear that he recognized the change in strike of the formations from north- south to east-west at the south end of the Las Vegas Range.

Spurr's suggestion that the "main fold of the Las Vegas Range,..., runs across the Las Vegas Valley and is probably to be found in the Spring Mountain Range on the other side...," is not acceptable in our present knowledge. Rowe's observations of stratigraphy and faulting are fairly accurate. F. C. Lincoln (1923) mentioned the Gass Peak Mining District in his early summary of mining operations of Nevada. From the most important mine, presently known as the Jung Bug Mine, zinc, gold, and silver ore was produced until abandon¬ ment in 1917. 3.

C. R. Longwell, in his careful reconnaissance mapping of the one-degree Las Vegas quadrangle, covered the Gass Peak quadrangle and accurately reported the main features of the area on the map by Bowyer, Pampeyan, and Longwell (1958), Several other papers by Longwell on the Las Vegas and ad¬ jacent areas have been published (see references). Longwell

(1960) first suggested the presence of the Las Vegas Valley

Shear Zone and made numerous other observations and comments in the literature that have been of great assistance to this writer. Southern Nevada geology has profited greatly under his study.

LOCATION AND DESCRIPTION OF THE STUDY AREA

The north half of the U.S.G.S. "Gass Peak" fifteen- minute quadrangle (Figure 1) was mapped during two months of field work in the summer of 1964. Gass Peak (V.A.B.M. 6943), the dominant topographic feature of the quadrangle (Plate I), is 15 miles north of Las Vegas, Clark County, Nevada. Maximum relief encountered is about 4000 feet. Elevations range from 3000 feet to 7000 feet, but only 1000 to 2000 feet of relief is present locally. The climate is dry, desertic, with temperatures often exceeding 100°F. in the summer and precipitation averaging

4 to 6 inches, annually. Vegetation is sparse in most of the area and ecologic zonation ranges from the Creosote

Bush to the transition between the Joshua Tree and Pinon- Juniper floral zones. Several fresh water springs are present 4 5

in the area, but most were dry at the time of study, and

those still active were contaminated by alkali saltso

STRATIGRAPHY

METHODS Because the emphasis in this study is on structural

geology, detailed description of the lithologic units recog¬ nized was not attempted* The writer briefly described units

in the field, for comparison along strike, and hurriedly measured some of ttTe Precambrian and Lower Cambrian units,

to compare thicknesses with other outcrops nearby* Dr* B* C. Burchfiel identified the various rock units in

the mapped area while accompanying the writer on reconnaiss¬ ance* Some of the units are incomplete and change thickness

drastically along strike, apparently because of "tectonic thinning", or replacement and omission of beds by faults within the formations* It remains to be determined whether

the stratigraphic section is more complete farther north • . . along the Gass Peak Thrust* GENERAL The rocks in the Gass Peak area represent some of the Precambrian and Paleozoic formations of the miogeosynclinal

facies of the southern Cordilleran Geosyncline* The lowest third of the upper plate of the Gass Peak Thrust is Pre¬ cambrian and Lower Cambrian orthoquartzites, sandstones, and

shales* A transition sequence of sandstone, shale, and lime¬ stone separates these terrigenous rocks from the dominantly

i 6.

STRATIGRAPHIC UNITS AGE THICKNESS V

Alluvium - Older Fanglomerates Quaternary ?

unc onformity —

Horse Spring Formation Tertiary ? (Miocene?)

unconformity

Permian- Bird Spring Formation Pennsylvanian— 5500+ MLssissippian

Monte Cristo Limestone* Mississippian- 1000+ Sultan Limestone Devonian (undiffer entiat ed )

—nonsequence —

Pogonip Group (lowermost) Ordovician 500+

Nopah Formation Late Cambrian 750+ Dunderberg Shale Member -

Middle-Late Bonanza King Formation; Cambrian 3000+

Carrara Formation; Early - Middle 675- Cambrian

Wood Canyon Formation Precambrian - 1350^ Early Cambrian (

Stirling Quartzite Precambrian 0-200+Iwi

—v*—fault —— FIGURE 2. Stratigraphic Units of the Gass Peak area, Nevada

t 7.

carbonate Middle and Upper Cambrian formations® Only the

lowermost Ordovician rocks are present in the area mapped, but they are succeeded by other Ordovician, Silurian,

Devonian, and Carboniferous formations northwest of the mapped area in the Sheep Range® The footwall consists of

almost 7000 feet of Devonian and Permo-Carboniferous rocks® One of these rock units, the Bird Spring Formation (Hewett,

1931) has been the subject of periodic discussion because it contains the elusive Pennsylvanian-Permian systemic boundary. Most authors agree that its lowermost beds are Mississippian. Unconformably overlying the Paleozoic rocks is a highly variable sequence of fine-grained carbonates, sandstones,

clay-stones, and volcanic tuffs of Tertiary (Miocene?) age.

Alluvial gravels and fanglomerates unconformably overlie all the older formations in the quadrangle.

FORMATIONS PRESENT

Precambrian Rocks Stirling Quartzite

The oldest rocks occurring in the Gass Peak quadrangle are beds of quartzite tentatively identified as the Stirling

Quartzite because of lithologic similarity to rocks described by Nolan (1929) as Stirling in the northwest Spring Mountains, and by their stratigraphic position below the Wood Canyon Formation. The Stirling crops out at the base of the upper plate of the Gass Peak Thrust throughout most of the

quadrangle, but its thickness ranges from zero to approxi¬ mately 200 feet. Because the base of the Stirling is a 8

fault there may be a considerable thickness of the Stirling missing. In the Spring Mountains, Nolan (1929) measured 3700 feet, and recently Hamill (personal communication) found 4200 feet of Stirling in part of the Spring Mountains west of Nolan's area.

"White and pink quartzite, with a few thin, silty inter¬ beds are the only rock types present in the Stirling

Quartzite. Most of the quartzite is coarse-grained to con¬ glomeratic and very hard. Some cross-bedding is present, but the most common feature of the Stirling is its highly • fractured, or even brecciated, condition, especially near

the thrust contact. The contact between the Stirling and the overlying Wood Canyon Formation is apparently conformable, with pebbly white quartzite of the Stirling grading into tan, coarse-grained quartzite and siltstone of the Wood Canyon.

Cambrian Rocks Wood Canyon Formation

The Wood Canyon Formation overlies the Stirling Quartz¬ ite and is recognized from descriptions in the Spring Mountains (Nolan, 1929; Burchfiel, 1964; Vincelette, 1964; and Hamill, 1965). At only one locality in the Gass Peak area is there possibly a complete section of Wood Canyon. One mile southwest of Quail Spring, the writer measured approxi mately 1350 feet of Wood Canyon beds, including approximately 100 feet of covered section in the middle. Elsewhere.along strike the Wood Canyon is thinner with different parts of rock units in the measured section apparently missing because of faulting. 9

The Wood Canyon is a highly variable formation con¬ sisting of alternating quartzites, sandstones, shales, silt-

stones, and a few thin beds of dolomite. The quartzites are thinner bedded, finer grained, and usually darker colored (maroon, purple, brown, and green) than those of the

Stirling. Shale beds become important near the top of the section as the coarser elastics become less common. A few, thin brown-weathering dolomites are present in the upper part of the formation. The contact between the Wood Canyon and the overlying Carrara Formation is chosen as the base of the first alternating sequence of thick bedded limestone and green shale above the quartzites of the Wood Canyon, The Zabriskie Quartzite, usually found at the top of the Wood Canyon Formation, is not present in the Gass Peak Area.

No fossils were found in the Wood Canyon in the Gass Peak area, but Nolan (1929) and Hazzard (1937) found Early Cambrian trilobite remains in the upper half of the formation.

Burchfiel (1964) regarded the lower half of the Wood Canyon as possibly Precambrian because of the lack of fossils there. In the Gass Peak area, the Precambrian-Cambrian boundary is not defined sharply and it is arbitrarily placed as somewhere near the base of the Wood Canyon Formation. Carrara Formation Following Cornwall*s and Kleinhampl's (1961) work at

Bare Mountain, 50 miles northwest of the Gass Peak area, and Burchfiel*s (1964) statement concerning subdivision of these beds, the Carrara here applies to "the transitional sequence 10.

between the Lower Cambrian quartzite and the Middle Cambrian carbonate formations." In the Gass Peak area, the Carrara

consists of alternating green and olive-green shales and thin (1 to 5 foot), interbedded silty limestones in the lower half, and more massive limestones with less shale in

the upper half. The limestones are commonly laminated and silty which causes them to weather buff or light brownish-

gray. Pisolitic structures, similar to the algae Girvanella, are common in the limestones of the Carrara. In a section above Quail Spring, northeast of Gass Peak, the writer measured approximately 675 feet of Carrara. This

is notably thinner than the 1500 feet (Nolan, 1929) reported in the northwest Spring Mountains. There is surely some

omission of beds by faulting, because most of the shales are slickensided and faults are present in the measured section.

Bonanza King Formation The thick sequence of limestone and dolomite overlying the Carrara Formation is known to be the Bonanza King For¬ mation from the work of Palmer and Hazzard (1956) in the Providence Mountains, California, and from Barnes and Palmer (1961) in the Nevada Test Site.

Barnes and Palmer (1961) have established the age of the Bonanza King in southern Nevada as Middle and Late Cambrian where they measured over 5000 feet of section. In . the Spring Mountains, thicknesses of 1500 to 2800 feet have been reported by Secor (1962) and Vincelette (1964) respec¬ tively. The writer measured approximately 3000 feet of 11.

Bonanza King just east o£ Fossil Ridge, which compares well with the 3000 feet measured by Burchfiel (1964) in the Specter Range.

Two members can be recognized in the Bonanza King For¬ mation. The lower member,- probably equivalent to the

Papoose Lake Member of Barnes and Palmer (1961), consists of interbedded limestone and dolomite, mottled dark- and light-

gray limestone and dolomite, and silty buff-weathering lime¬ stone with thin shaly interbeds. The upper member, probably

equivalent to the Banded Mountain Member of Barnes and

Palmer (1961), consists of a basal silty, reddish-brown weathering limestone and shale unit, and higher, banded

light-and-dark gray, relatively pure limestone and dolomite. The contact between the Bonanza King and overlying Nopah Formation is in an alternating sequence of fairly pure

limestone and olive-green shale. Top of the Bonanza King is mapped as the top of the highest massive, fairly pure lime¬

stone, below the first green shale. Nopah Formation The striking, banded white-and-black south face of

Fossil Ridge is a beautiful exposure of the beds of the Nopah Formation. Near the northwest corner of the mapped area is an incomplete section of Nopah. A little farther east the missing basal member is exposed. Approximately 750 feet of Nopah is present in the Gass Peak area. Again there is a significant thinning of the Nopah between the

Gass Peak area and areas farther north and west. The top of 12

the Nopah Formation is placed at the top of the highest

massive, dark gray dolomite that conformably underlies the thin-bedded, silty limestone of the basal Pogonip Group de¬ scribed below.

The Nopah Formation can be divided into two members. The upper member consists of thick-bedded, alternating dark gray and light gray, coarsely crystalline dolomite in the upper part and rough-weathering impure limestone in the lower part. The upper member stands out as a cliff above the lower

shaly slopes formed by the Dunderberg Shale Member. The Dunderberg Shale Member is 200 feet thick, consisting of beds of hard, fissile, olive- to bright-green shale in the lower half with interbedded light gray and buff, silty, wavy-bedded limestoneso The limestones become thicker (3 to 10 feet) upward and the shale becomes less common. About

175 feet above the base of the Dunderberg is a zone of silty, cross-bedded limestone with flat limestone pebbles. Several of the limestone beds in the member contain rounded calcareous nodules enclosing parts of fossil trilobites. The contact between the Dunderberg Shale Member and the upper member is transitional. The top of the Dunderberg is mapped as the base of the first massive, cliff-forming limestone that con¬ tains only minor silty impurities. Some geologists (Burchfiel, 1964; Barnes and Palmer, 1961) have mapped the Dunderberg Shale as a separate for¬ mation in areas north and west of the Spring Mountains. How¬ ever, workers in the central and northern Spring Mountains • 13

and in the Nevada Test Site have chosen not to differentiate

this shale from the overlying Nopah Formation, following Hazzard's precedent from the Nopah Range, California. The writer follows the most recent stratigraphic conclusions of

Barnes and Christiansen (1965) and does not differentiate

the Dunderberg as a separate formation.

No fossils were found in the dolomitic upper member of the Nopah. On the basis of its stratigraphic position bet¬ ween the Upper Cambrian Dunderberg Shale Member and Lower

Ordovician of the Pogonip Group, the upper member has been assigned an age of Late Cambrian with possibly the uppermost

beds being Early Ordovician. Abundant fossil trilobites and tiny, phosphatic, linguloid brachiopods are present in the basal Dunderberg Shale Member. Dr. J. L. Wilson has identi¬

fied several of these trilobites as:

Dunderbergia, 2 spp.

Housia ?, free chee.k, free cheek undetermined these fossils are clearly Upper Cambrian and correlate with the Dunderberg fauna (Palmer, 1960).

Ordovician Pogonip Group Only a few hundred feet of beds belonging to the lower Pogonip Group crop out in the mapped area, but higher Ordovician and Middle Paleozoic rocks are present immediately north of the area. The writer did not attempt to measure the exposed Pogonip rocks because of the numerous faults present. 14.

Thin-bedded, very silty limestones and minor siltstone

are present at the base of the Pogonip. The limestones be¬

come thickly bedded, in the upper part of the exposed Pogonip

but still are silty with thin, shaly interbeds.

Devonian-Mississippian rocks (undifferentiated)

Monte Cristo Limestone and Sultan Limestone

The only exposures of Devonian and Mississippian beds

in the Gass Peak area are in the core of a tight, eastward

plunging anticline at the southeast foot of Gass Peak and in

discontinuous outcrops along the base of its south face.

The attitude of these beds is variable and they are cut by

numerous faults, which makes reconstruction of a measurable

section very difficult. The presence of these beds, as indi¬

cated by Longwell (1958) is acknowledged, but the writer

attempted no description of the section. The formations

recognized are the Mississippian Monte Cristo Limestone and

the Devonian Sultan Limestone.

Mississippian-Pennsvlvanian-Permian rocks

Bird Spring Formation

The bulk of Gass Peak itself is underlain by the thick

sequence of carbonate rocks known as the Bird Spring For¬

mation. Hewett (1931) named this formation after the Bird

Spring Range 50 miles to the south, where he described the

type section. In most areas, the Bird Spring is a very un¬

handy unit as a single formation for it comprises several

thousand feet of beds that span possibly part of Mississippian, all of Pennsylvanian, and part of Permian time. Attempts have 15

been made (Longwell and Dunbar, 1936; Bissell, 1962;

Rich, 1961, 1963; Langenheim, 1962) to subdivide the Bird Spring lithologically and to zone it faunally. Almost all

workers agree that the Bird Spring should be elevated to group status, but they generally disagree on the placing and naming of subdivisions within this group. There are few

beds in the Bird Spring that are useful as markers for mapping structure, and the cyclic nature of the bedding in parts of the formation is bewildering, making fault displacement very

difficult to determine.

Beds of quartzite, sandstone, shale, and very silty limestone, weathering a characteristic reddish-brown and yellow in outcrop, are recognizable as the basal Indian

Springs Member of Longwell and Dunbar (1936). These beds, approximately 100 feet thick, separating the rest of the Bird Spring from the Mississippian Monte Cristo Limestone, are the most distinctive part of this formation and have been

mentioned by every writer describing the Bird Spring For¬

mation in this part of Nevada. The terrigenous clastic con¬

tent and texture of this unit and the fact that it varies in

thickness along strike suggest the presence of a disconfor- mity between Monte Cristo and Bird Spring beds. Above the basal member of the Bird Spring, the formation consists of cherty limestone and dolomite, massive, cliff-forming lime¬ stone, buff-weathering sandy, platy limestone, and ledge-

forming massive limestone. 16

No attempt to measure the thickness of the Bird Spring was made; but estimates from air-photos indicate approximately 5500 feet of Bird Spring beds crop out where the section is most complete comprising Gass Peak, Rich (1961), by his work with fusulinids, has shown that the top of the 7000 foot, incomplete section he measured in the Spring Mountains is Permian (Leonard) in age. Lime¬

stone collected from the Bird Spring Formation on Gass Peak contains fusulinids identified as Pseudofusulina sp„, also Permian (Leonard) in age.

Tertiary rocks Horse Spring Formation A variable section of Tertiary rocks crops out beneath the bahada north of Gass Peak and on the south slope of Fossil Ridge, The rocks consist of pastel-colored beds of limestone, claystone, dolomite, sandstone, tufa, and volcanic tuff in various stages of alteration. All of these rock types overlie a basal conglomerate deposit, which is composed of debris from Paleozoic rocks in a matrix of porous, clayey limestone.

The outcrops of these rocks are patchy and the same beds do not always crop out in adjacent exposures, even though their deformation is slight. The rocks appear to be a local basin deposit which are difficult to correlate with other areas. Longwell (1958) has designated these beds as the

Horse Spring Formation that he named and described from the Muddy Mountains 30 miles to the east. The writer did not 17

attempt to verify this correlation in the field, and did not measure or reconstruct a complete sequence of these beds.

Nevertheless, there is a striking resemblance to Longwell's measured sections in the Muddy Mountains.

Though Longwell reports no fossils from beds of the

Horse Spring the writer collected a few fresh water gastro¬ pods and some non-marine ostracods in the Gass Peak area.

Attempts to identify these specimens have been unsuccessful

thus far. Longwell (1928) tentatively dated the Horse Spring

Formation as Miocene in age because of 1) similarity of rock

types in that formation with other sequences of lacustrine beds and tuffs (feted as Miocene in southern Nevada, California,

and Arizona, 2) similarity of all of those beds in their

stage of deformation, and 3) the presence in all of those

areas of unusual deposits of magnesite and borax. Rubey and

Callaghan (in Hewett, 1936) found Upper Cretaceous plant re¬ mains in the underlying Overton Fanglomerate in the Muddy

Mountains, and assigned that age to both the Overton and the

Horse Spring. Van Houten (1956) correlated the Horse Spring

Formation to the Claron Limestone and Wasatch Formation of

southwestern Utah "on the basis of general stratigraphic position and lithologic character," and suggested an age of

Eocene for these beds. Armstrong (1963) obtained K-Ar dates

on a sample of Horse Spring tuff below the lowest limestone

and 200 feet above the Overton Fanglomerate. His conclusion was that the Horse Spring is Miocene. 18. 19

Until firm dates can be established by faunal evidence and radiometric dating (in progress), the writer accepts the correlation of the lake beds and tuffs of the Gass Peak area with the Horse Spring Formation and, thereby, a Miocene age.

Quaternary rocks Older-and Younger Alluvium

At least two stages of alluvium have been deposited in the Gass Peak area, both resting unconformably on the Miocene and older rockso The older gravels and fanglomerates flank all the presently high areas. These gravels are firmly cemented and retain some of their former extent» The present drainage system has dissected these old fans and is depositing a new sequence of alluvial gravels that are uncemented and much less resistant to erosion. 20

STRUCTURAL GEOLOGY

REGIONAL SETTING The Gass Peak area lies in a great belt of overthrusting extending from southwestern Wyoming to southern Nevada and

adjacent California. The overthrusts moved rocks of the mio- geosynclinal part of the Cordilleran Geosyncline to the east

for a distance of perhaps 25 miles. The great overthrust belt can be traced into southeastern Nevada, where all the thrusts appear to end at the Las Vegas Valley (Figure 3), and no positive continuation of any one fault can be observed

across the valley. Longwell (1960) suggested that Las Vegas Valley is the locus of a major strike-slip fault zone, and that the "diverse structural patterns" in the ranges north and south of the valley are drag features produced by lateral

movement oh this right-lateral shear zone (Figure 3).

Burchfiel (1965) modified Longwell* s idea of separate movement between "basement" and "cover" rocks. Having mapped areas straddling the trend of the Las Vegas Valley

Shear Zone, he concluded that, north of the Spring Mountains

in the Specter Range, movement on this shear zone by basement

rocks was compensated in the sedimentary cover rocks by bending and folding and by high-angle faulting; while, farther southeast, along Las Vegas Valley, lateral displace¬

ment of the cover rocks was more important. Burchfiel fur¬

ther suggested that the thrust at Wheeler Pass, in the Spring Mountains, correlates with the thrust in the Las Vegas Range 21.

(the Gass Peak Thrust), implying 27 miles of lateral shift between the two. This correlation is based on the coinci¬ dence of formations on the soles of these thrusts. Nolan (1943) realized that the time of folding and thrusting in the eastern Great Basin was not merely an ex¬ tension of the classical Nevadan (Jurassic) orogeny of

California or the Laramide (Cretaceous to Eocene) orogeny in the Rocky Mountains. Recently, Armstrong (1963) dated various rocks involved in the overthrusting and emphasized the need to distinguish this period of deformation as an entity involving the Eastern Great Basin during Latest Jurassic to Earliest Tertiary time. He coined the term "Sevier Orogeny" to illustrate his point, and described the various features along this belt and their relations in time.

STRUCTURAL FEATURES OF THE GASS PEAK AREA Gass Peak Thrust

General Armstrong (1963) suggests continuity of a major over¬

thrust, bringing lower Cambrian rocks over upper Paleozoic rocks, from northwestern Utah to southern Nevada, as the main thrust of the Sevier Belt. Whether it is necessary to propose a single thrust of this great extent is subject to

some question, but the suggestion cited does point to evi¬ dence of similar displacement on a fault zone of more than

local importance. 22P

The Gass Peak Thrust, southernmost element of this

regional complex, is exposed north of the subject area of this report, in the Hayford Peak quadrangle, and can be traced for ten miles to the north before it disappears under an alluvial cover. Its presence is implied still farther

northward into Lincoln County, Nevada, by the outcrop pattern

of the rocks there (Bowyer et al., 1958). The most important structural feature of the Gass Peak quadrangle is the Gass Peak Thrust, which is continuously exposed for ten miles across the mapped area. Many less im¬ portant folds and faults are directly related to movement on

this overthrust. Stratigraphic Displacement At the base of the upper plate of the overthrust is Pre- cambrian Stirling Quartzite. The youngest beds in the foot- wall are the Permian upper Bird Spring Formation. One of the most significant fusulinids from these upper beds in the footwall was tentatively identified as Pseudofusulina sp. Christy (1958) and Rich (1961) found a species of Pseudo¬ fusulina in beds approximately 6300 feet above the base of the Bird Spring Formation, which they place in the Permian, Leonard, Schwagerina Zone. Because the thrust overrides beds high in the Bird Spring Formation, the stratigraphic throw on the Gass Peak Thrust is approximately 18,000 feet, if minimum thicknesses are used to calculate displacement.

It is interesting to note that Vincelette (1964) was able to date the highest beds of the Bird Spring Formation in 23.

the footwall of the Wheeler Pass Thrust in the Spring

Mountains as also Permian, Leonard, Schwagerina Zone. Thus

the Wheeler Pass and Gass Peak thrusts cut the footwall

sections at about the same stratigraphic position.

Geometry

The surface trace of the Gass Peak Thrust is one of its

most curious features. From the north boundary of the Gass

Peak quadrangle southward, the fault trace changes strike

from almost due-north to due-west. The subject area of this •

report was chosen to include this arcuate segment. This

curvature of the fault trace is no topographic accident.

Inspection of the strike symbols of Plate I in the hanging wall and in the foot wall, along the fault, reveals that

attitudes of the formations there change also, remaining paiaLlel to the fault trace throughout the bend. If this were

a very low angle fault, the above relationships might still be related to topography, but the fault plane is observed to

dip, westward to northward, 25 to 50 degrees. This indicates a real curvature of the fault plane.

Footwall

There is a brecciated zone about 10 feet to 20 feet wide on either side of the fault, which commonly obscures the fault. This "gouge zone" is better developed along the north slope of Gass Peak than it is farther northeast.

The Bird Spring beds beneath the fault are in various attitudes. In some places the beds dip at 25 to 40 degrees toward the fault, at other places they dip steeply (40 degrees) 24 away from the fault, but beneath the fault in most places the beds are overturned in a tight synclinal fold* The over¬ turned beds are particularly common along the section of the thrust southwest of Quail Spring and north of Gass Peak* Evidence for the relative eastward direction of movement of the hanging wall during thrusting is established by the numerous minor folds in the beds of the footwall immediately below the fault* Most of these folds are too small to show on Plate I* The overturned syncline mentioned above may be only a drag feature, but the minor anticlines and "accordion- folds" much more likely resulted from compression in the direction of overthrusting. Evidence to be discussed below indicates that the thrust surface was originally uncurved, and that there was no southward component of overthrust move¬ ment. In one deep cut on the north slope of Gass Peak

(see Fig. 5), the minor "accordion folds" extend several hundred feet up the dip of the beds. As is visible in

Figure 5 these folds die out abruptly upward in section. Clearly the shaly, thinner-bedded part of the section res¬ ponded differently to the compressive force than did the thicker-bedded, more massive limestones above. Other features of the footwall rocks related to the thrusting will be discussed below. Hanging Wall The Gass Peak Thrust cuts approximately the same portion of the stratigraphic section all along its strike in the Gass Peak quadrangle, following just below a massive bed of con-

ft Figure 5„ Small-scale folding in the Bird Spring Formation adjacent to the Gass Peak Thrust. Tight assymetric folds occur in thin, silty limestone. Higher, more massive beds are unfolded. 26. glomeratic white quartzite. This unit resembles the massive basal member of the Stirling Quartzite of the Spring

Mountains area (Hamill, 1365, personal communication). The beds of the hanging wall dip at a slightly lower angle than does the thrust plane itself. This indicates that the fault cuts these beds at a very small angle, probably everywhere less than 10 degrees. The thickness of the Wood Canyon Formation varies greatly along strike, suggesting some intraformational thrusting, or "tectonic adjustment" in the hanging wall. In fact, numerous small folds and thrust faults, mostly too small to show on the geologic map, can be seen in one or two deep stream cuts or prospect pits. These all indicate eastward override in the same direction as movement of the main overthrust. / Decollement Faulting The features of the hanging wall mentioned above suggest that the Gass Peak Thrust probably remains at very nearly the same stratigraphic level and actually becomes flatter at depth, passing into a zone of decollement. This implies that the thrust never involves the crystalline basement rocks and is confined to the sedimentary cover, but no ex¬ posure of the Gass Peak Thrust shows any of its deeper geo¬ metry.

Several other studies in southern Nevada have indicated / the presence of decollement thrusts. Longwell (1928) recog¬ nized the Muddy Mountain Thrust as such a feature. Nolan’s 27.

ideas of the Wheeler Pass Thrust as a decollement fault in the northwest Spring Mountains, though challenged in detail by later workers (Livingston, 1964; Vincelette, 1964;

Burchfiel, 1965), are largely still valid. Rowe (in Spurr,

1903) and several later workers (Glock, 1929; Longwell, 1926, 1960; Secor, 1962) described the Keystone Thrust in the southern Spring Mountains as a similar structure. Burchfiel (1965) suggested that all of the five major thrusts in the Spring Mountains are de^collement-type structures.

Like the Gass Peak Thrust, none of the faults mentioned abcv e brings basement rock to the surface. This analogy streng¬ thens the hypothesis that the Gass Peak Thrust is probably a near bedding-plane fault at depth.

Other Features

Two other features of the thrust deserve further attention because of their unusual nature. Along the northern one-third of the Gass Peak Thrust in the mapped area the thrust sheet is imbricated and a small window, in the thrust sheet exposes Bird Spring limestone.

Longwell (in Bowyer, Pampeyan and Longwell, 1958) mapped Precambrian Johnnie Formation below the Stirling

Quartzite near the northern boundary of the Gass Peak quad¬ rangle. This seemed anomalous to the writer and necessitated additional investigation. An alternate explanation of this feature is required by the fact that the beds cropping out below the Stirling are shales, siltstones, and limestones of the Carrara Formation, not of the Johnnie Formation (see 28.

Plate I). The rocks are extremely fractured and outcrops are poor because the shale covers the slopes as small platy frag¬ ments, but a small exposure near the foot of Quartzite Hill

shows the highly sheared and cleaved condition of the beds. Identification of these beds was based on the presence in

some of the thin limestones of Girvanella pisolites and a coquina of trilobite free cheeks and other remains, similar

to those fossils found in the measured section of Carrara beds at Quail Spring. Approximately 150 feet of Carrara is present here. The Stirling Quartzite that overrides the imbricate slice of Carrara along Hills 6560 and 6134 is identical to the Stirling at the base of the main overthrust farther south. On the slopes east of Quail Spring Road beds of Stirling and Carrara dip more steeply than the overthrust plane, as indicated by the very irregular trace of the con¬ tact of these beds with the underlying Bird Spring limestones.

Beds of the upper plate dip northwest and west 40 to 80 degrees but the overthrust plane dips less than 30 degrees in the same direction. The sequence of Stirling and Carrara seems to be repeated two or three times in these steeply dipping beds. This relationship is explained by the presence on Quartzite Hill of beds of Stirling Quartzite dipping 10 to 20 degrees westward, which represents Stirling continuous with that overlying the Carrara of the imbricate slice on Hill 6560 and near the axis of an anticline that strikes north-south. The beds of Stirling and Carrara just south of 29.

Quartzite Hill are remnants of low-angle normal fault blocks on the west limb of the anticline which have been faulted down to the east to produce the unusual relations described above. The subsurface conditions that produce such an imbri¬ cation of the thrust sheet are, of course, highly conjectural. The writer considers the explanation offered in cross-section A-A' to be the best of several interpretations, but it is probably an oversimplification. The presence of a wedge of thrust sheet beneath the main thrust is indicated by the de¬ creased dip of the Bonanza King Formation over the zone where this wedge would add to the thickness of the overthrust mass. Further study in the Hayford Peak quadrangle, where this imbrication continues, may shed additional light on the exact structure of this feature. The imbricate slice of Carrara is eroded down in one place to expose underlying beds of Bird Spring limestone. These beds, 2k miles northeast of Quail Spring, just north of Hill 6134 and straddling the jeep trail, are 100 feet in total thickness, and contain productid brachiopod fossil fragments similar to some seen in the Bird Spring at other localities. The position of this outcrop in a topographically low position, relative to parts of the thrust sheet all around it, its fairly undisturbed bedding, the conformity of the bedding attitude to that of other Bird Spring outcrops near¬ by, and the visible overthrusting of the west and north sides of the window by the Cambrian beds, all support the interpretation of this feature as a window. 30

Structural features related to thrusting General A rose diagram of faults and fold axes in the Gass Peak area would be almost completely meaningless to illustrate trends of fracture in the rocks because almost all of the structures in the area have been rotated to some degree by movement along the Las Vegas Valley Shear Zone. Therefore, it is more useful to discuss the features noted in relation to the major deformations that produced them. Footwall Features

Broad folds

Northeast of East Pass are a number of large, open anticlines and synclines in the Bird Spring Formation whose axes parallel the strike of the Gass Peak Thrust (cross- section B-B’)« These folds are commonly expressed topo¬ graphically as long, gently arcuate ridges or valleys. South of these gentle folds, the beds tend to resume a fairly uni¬ form northwestward dip. Presumably, this folding is related to compression during the thrust faulting.

Reverse faults Southwest of East Pass, on Gass Peak itself, there is a somewhat different response to the compressive force from the north and northwest. Instead of broad folds, numerous reverse faults are present. Two very long, almost continuous, faults formed approximately parallel to the Gass Peak Thrust (cross-section C-C1). The throw on these and the many smaller 31 reverse faults shown on Plate I never exceeded a few tens of feeto No topographic scarps are formed by these faults. Some of the smaller reverse faults formed from tight folds broken through near their axial surfaces. A peculiar characteristic of the reverse faults is that the dip of the beds of the hanging wall is often less than the dip of the footwall beds, resulting in less repetition of section than might be expected on a reverse fault. This * peculiarity is more common on the north face of Gass Peak than on its south slope, because, once again, the Bird Spring beds tend to maintain a general north or northwestward dip.

Overturned folds

At Broken Hill (secs. 24, 25, 26, T. 18 S0, R. 61 E.) and to the east across East Pass (secs. 7, 18, T. 18 S.,

R. 62 E.), the Mississippian Monte Cristo Limestone and part of the Devonian Sultan Limestone crop out in the highly fractured cores of overturned anticlines whose axes plunge gently both east and west away from East Pass. This indi¬ cates that East Pass follows a fault zone older than the folds, and that the folds developed differently on opposite sides of that separation. The tightly folded, northeast trending anticline south¬ east of Dry Wash (cross-section B-B') is recumbent along at least half of its exposed length. In sections 8, 9, 16, 17, 20, T. 18 So, R. 62 E., beds of the Bird Spring Formation in the overturned limb of the fold form a spectacular outcrop, dipping near vertical to 15 degrees, upside-down (Figure 6). 32

Figure 6. Overturned beds south of Dry Wash. Bird Spring beds (PPMb) dip steeply northward (left in the picture) overturned. 33

This overturned limb is probably continuous in the section of Bird Spring beds south of Broken Hill (Figure 7). This large-scale folding probably involves very little of the strata beneath it and is probably only a superficial feature, because the beds quickly return to very gentle dips and normal attitude immediately southeast of the recumbent sector (see cross-section B-B')*

Normal Faults

On the south slope of Gass Peak, in sections 19-23,

27-29, To 18 S., R. 61 E0, the Bird Spring Formation and the Monte Cristo and Sultan Limestones are cut by two sets of east-west trending normal faults, one set downthrown-to-the- south, the other downthrown-to-the-north (Plate I)» South of Hill 5633 and in the vicinity of the June Bug Mine, faults dipping southward at approximately 50 to 60 degrees, repeat the northward dipping Monte Cristo Limestone and the basal beds of the Bird Spring Formation (Figure 8) 0 The June Bug Mine produced zinc ore (and some gold and silver) from dolomitized zones along fractures in the limestone (Lincoln in Hewett, and others, 1936). Numerous other prospect pits are evident along some of the fault lines in this part of the mapped area. In sections 19, 20, 28, 29 the faults dip northward at 25 to 30 degrees and cut beds dipping southward (Figure 9)„ The low-angle normal faults are more numerous and have less throw than the steeper faults. Figure 7. Overturned beds south of Broken Hill, Bird Spring beds (PPMb) dip steeply northward (left in the picture) overturned. 35

Figure 8. Normal fault dipping southward (left in the picture), and repeating the Monte Cristo Lime¬ stone (Mm) and basal Bird Spring Formation (PPMb). 36

Figure 9. Low-angle normal faults dipping northward (right in the picture) 40°, displacing beds of the Monte Cristo Limestone (Mm; and Sultan Limestone (Ds) down-to-the-north. 37.

The controlling structure producing both sets of faults is a large assymetric anticline, now obscured by the numerous faults. Longwell (1945) described much larger, but similar, low-angle normal faults on an anticline in ranges northwest of Gass Peak. Oppositely dipping faults probably formed near the crest of the Gass Peak anticline, those on the north limb dipping more steeply thanthose on the south limb. Hanging Wall Features

High-angle reverse faults High-angle reverse faults are a common structure on the northeast trending ridges of Bonanza King Formation, east of the Yucca Forest. The fault planes generally parallel the strike of the beds (see Plate I) and have displacements of only a few feet. The faults curve westward with the strike of the beds and are clearly related to the same compression that produced the Gass Peak Thrust. On the north side of Fossil Ridge, a steep anticlinal fold is broken-through along the axial surface by a reverse fault, repeating some of the Pogonip section. Most of the hanging wall is eroded, but the faultline scarp remains to mark the trace of this fault that dips northward 40 to 60 degrees, and cuts the Pogonip, Nopah and Bonanza King rocks along the length of Fossil Ridge. The fault trace parallels the trace of the Gass Peak Thrust and shows the same cur¬ vature in plan suggesting large scale bending of structures after faulting along Fossil Ridge. 38.

Broad folds Near the north edge of the mapped area, two southward plunging folds in the Bonanza King Formation parallel the faults mentioned above. The folds may be related directly to the thrusting and reverse faulting, but similar features are often cited in cases of decollement thrusting when a fault cuts across a competent horizon and follows a higher incompetent one (King and Ferguson, 1960; King, 1960) <, If this is the case here and this is a "step fold", then the

Gass Peak Thrust must not follow the upper Stirling Quartzite very far down dip before it changes to a lower stratigraphic level.

The anticlinal fold may also be attributed to drag on the upthrown side of a large, intermontane, normal fault in Yucca Forest Valley. This possibility has some merit because

Longweli (1945) mapped a large, down-to-the-west, normal fault, just north of the Gass Peak quadrangle that is properly oriented to produce a high scarp like that on the west flank of the Las Vegas Range. Structures related to the Las Vegas Valley Shear Zone

General Right-lateral movement on the Las Vegas Valley Shear Zone (Figure 3) is reported to have been in excess of 25 miles (Longweli, 1960; Burchfiel, 1965)., The large-scale physiographic elements of the areas adjacent to that zone of strike-slip movement certainly suggest drastic drag effects 39

corresponding to the right-lateral displacement. Neverthe¬

less, one must seek positive evidence for such drag in the deformational pattern of each such element. The whole length of the Las Vegas Range south of Latitude 36° 30* N may be considered one drag feature* Its

westward curvature is strongly opposed to the overall north- south trend of Great Basin ranges. Extension Faults

The most convincing structural evidence for the arcuate trend of the Las Vegas Range being a drag feature is a

pattern of normal faulting that transects other features re¬ lated to the thrusting.

Figure 4 is a simplified diagram of only the important folds and faults mentioned above* Fault I strikes almost east-west. It cuts the Lower Bonanza King and may continue

into the Carrara and Wood Canyon sections* Throw is down- to-the-southwest and varies from the few tens of feet near

the middle of the trace to almost zero at both ends. Fault II strikes S 60°E, has a displacement of approxi¬

mately 100 feet, and cuts the lower half of the exposed

overthrust sheet. Faults I and II also mark slight, but abrupt, changes in the strike of the beds from north-south,

to S 30°W, to S 45°W (see Figure 4). Fault III is a large structure that cuts the entire ex¬ posed overthrust sheet and may continue through East Pass.

This fault cuts the thrust fault trace one half mile north¬ east of Gass Spring and is expressed at the east end of 40

O 41

Fossil Ridge as a steep monoclinal flexure in the Uppermost Bonanza King section that produced bedding plane slip in the

Dunderberg Shale Member of the Nopah Formation (saddle bet¬ ween hills 5316 and 5775). Fault III may account for the

fact that most of the Wood Canyon, all of the Carrara and all of the resistant Bonanza King Formation do not crop out

between the north slope of Gass Peak and Fossil Ridge. Fault III is largely concealed, but it probably strikes approximately S 30°E and has a dip-slip displacement of several hundred feet. This fault may actually be a narrow

fault zone of several smaller faults. Faults IV A and IV B displace parts of the Bird Spring Formation down-to-the-west. As mentioned earlier, displace¬

ments in these limestones are difficult to determine, but Fault IV B does not cut the overthrust sheet and Fault IV A

displaces it only slightly. Fault V is inferred from topography, an apparent offset

of Bird Spring strata, and the absence of rocks of the thrust sheet at the north foot of hill 4366. Faults IV A and B and Fault V strike almost north-south. These normal faults, arranged in a radiating pattern normal to the strike of the Gass Peak Thrust and the general

strikes of the beds, probably formed as the south end of the

Las Vegas Range was rotated westward along the Las Vegas Valley Shear Zone and subjected to extension.

Backthrusts Backthrusts present in the Gass Peak area also support / the interpretation of bending caused by drag on the Las Vegas 42

Valley Shear Zone. At hill 4475, secs. 7, 18, T. 18 S,,

R. 62 E., and in the Broken Hill area to the southwest, high-angle, reverse faults dipping south and southeastward, 40 degrees to 60 degrees,, complicate the overturned folds. These backthrusts may be related to the thrusting, but more

likely are related to the same stress system that produced the normal faults discussed above; that is, they displace

beds that were involved in the tight folding of this area and are probably, therefore, post-thrusting structures.

A similar backthrust is developed in the Wood Canyon Formation along the north foot of Gass Peak. There the

quartzites of the thrust sheet are abruptly truncated, along strike, by a fault dipping steeply southeast to south under Gass Peak. The trace of this fault is marked in the field by steeply dipping, highly fractured quartzite and is so

obvious that it forms a clearly defined line on the air- photos of the area. The presence of this backthrust (see cross-section C-C') explains why an apparently continuous outcrop of the lower Wood Canyon Formation is exposed west of Gass Spring but the rest of the thick lower and middle Cambrian section is not exposed. Probably, most of the rocks forming Gass Peak and the adjacent thrust sheet were first faulted down by Fault III, but Gass Peak and the toe of the thrust sheet were subsequently faulted up-to-the-north to their present position.

To clarify the explanations offered for evidence of drag structure in the Gass Peak area, it is worthwhile to 43 consider the strain that would have produced features like those described above. Figure 4 schematically shows the de¬ formation involved here if one assumes that the Las Vegas Range did not hook-around to the west before the time of movement on the Las Vegas Valley Shear Zone. A rectangle ABCD would have been deformed to rhomb ABC’D1 in producing the present structure. This deformation involves extension in an east-west direction producing the normal faults dis¬ cussed above, and north-south compression, resulting in the back-faulting also described above.

Normal Faulting There are numerous smaller normal faults in the Gass Peak area. It is difficult to categorize these as associated with any particular period of faulting. If the above sugges¬ tion of normal faulting as a drag feature is correct, there were at least two periods of deformation when normal faults could have developed: 1) at the time of movement on the Las

Vegas Valley Shear Zone, and 2) during the almost continuous block faulting of later Tertiary time.

The normal faults cutting Fossil Ridge are favorably oriented to be associated with the drag movement, but they also have the same general orientation as many larger Tertiary normal faults in this part of the Great Basin. These faults are almost vertical and have strikes ranging from north- south to S 40°W. Certainly some Tertiary normal faulting is required to produce the recent uplift demonstrated by the ex¬ istence of the two stages of alluvium deposition described earlier. 44

Late Tertiary deformation The Tertiary sediments and volcanics that underlie much of the area between Fossil Ridge and Gass Peak dip as steeply as 30 to 45 degrees and contain minor folds* These beds dip toward the low valley from ridges of Paleozoic rock, forming

a large syncline whose axis trends east-west. The southern¬ most outcrops of these Horse Spring beds abruptly terminate at the steeply dipping backthrust in the Wood Canyon beds

mentioned above, and are folded there into minor anticlines (indicated by the patterns of strikes and dips on Plate I and by cross-section C-C')* The only late Tertiary deformation

commonly recognized in this part of the Great Basin is block

faulting* The slight deformation of the Horse Spring beds apparently related to movement on the Las Vegas Valley Shear

Zone clearly implies that some of this movement occurred as late as late Miocene time.

Gravity Sliding In the area of Quail Spring is an exposure of Bird Spring limestone one half mile northwest of the trace of the

Gass Peak Thrust. The limestone, dipping 30 degrees northward

is in contact, along a brecciated zone, with Stirling Quartzite. Together, the two formations have the same relation as they do at the Gass Peak Thrust contact, but they are displaced almost

three-quarters of a mile from their supposed normal position. This unusual outcrop mass is interpreted as a gravity slide block which retained its overall character while sliding northwest to its present position. The contact of this slide 45 block with the underlying formations is difficult to see clearly, but the manner in which this mass rests with indis¬ cretion on several formational contacts, the flat contact of the slide mass with underlying formations, the position of the block in a topographic low adjacent to a normal fault, and the nearby location of a source for the mass, indicate that this is, in fact, a gravity slide block and not a window in the thrust sheet. The isolated blocks of Stirling Quartzite, along the Quail Spring Road north of Gass Spring, are probably also gravity slide blocks. These disconnected, highly brecciated, outcrops of massive, pebbly, white quartzite protrude through alluvial gravel, and seem completely unre¬ lated to any nearby formation. A likely source for such slide blocks would have been the Stirling Quartzite at the northeast corner of Gass Peak in sec. 4, T. 18 S., R. 61 E.

The arrangement of the several smaller quartzite outcrops in a string-of-beads pattern downhill from the largest out¬ crop of Stirling Quartzite suggests the source for the gravity slide blocks. The writer prefers this explanation for its simplicity and because there is no other pertinent evidence to explain the nature of these outcrops. 46

TIMES OF MAJOR DEFORMATIONS Thrust faulting

The movement of the numerous thrust faults in Southern

Nevada is poorly dated except in the Muddy Mountains 30 miles east of the Gass Peak area. Longwell (1936) found that, in the Muddy Mountains, the overthrust sheet rests partly on the Jurassic Aztec Sandstone and partly on the Cretaceous Willow Tank Formation. Based on this evidence, Longwell suggested that the thrusting there took place in late Cretaceous or early Tertiary time. Secor (1962), working in the Spring Mountains, noted cobbles eroded from the Wood Canyon Formation on the erosional surface of the Aztec Sandstone that is overridden by the

Keystone Thrust (Figure 3). He implies that since the Wheeler Pass Thrust, several miles northwest of his area, was the only thrust in the Spring Mountains to bring Wood Canyon rocks to the surface, it must predate the Keystone Thrust. Longwell (1960) proposed that the Keystone and Muddy Mountain thrusts were offset equivalents across the Las Vegas Valley Shear Zone. Burchfiel (1965) proposed the same relation bet¬ ween the Wheeler Pass Thrust and the Gass Peak Thrust, and suggested 27 miles of offset between them. If these analogies are correct, the Gass Peak Thrust predates the Muddy Mountain Thrust, but is certainly related to the same deformation and is probably a Cretaceous feature. 47

Strike-Slip Faulting Longwell (1960) offers evidence in the Spring Mountains for the idea that thrust faulting and strike-slip movement in Las Vegas Valley were partly contemporaneous. In the Gass Peak area no such evidence is present. In fact, it seems that faulting on the Gass Peak Thrust definitely preceded drag deformation associated with strike-slip faulting. In several instances, as discussed earlier, features produced by the drag transect structures surely produced by the thrust faulting. However, the Gass Peak Thrust was probably de¬ veloped early among the southern Nevada thrusts; and the strike-slip faulting could have been contemporaneous with later faults like the Keystone Thrust. Minor deformation of the Horse Spring beds north of Gass Peak (cross-section

C-C*) may indicate that at least some movement on the strike- slip fault in Las Vegas Valley occurred as late as Miocene. 48

SUMMARY

The present study has recognized the stratigraphic units present in the southern Las Vegas Range and mentioned corre¬ lations with nearby areas. Latest Precambrian Stirling

Quartzite; Precambrian and Cambrian Wood Canyon Formation, the Cambrian Carrara, Bonanza King, and Nopah Formations; and lowermost Ordovician Pogonip beds are thrust eastward over the Pqrmo-Carboniferous Bird Spring Formation, the

Mississippian Monte Cristo Limestone and the Devonian Sultan

Limestone. Beds tentatively identified as the Miocene Horse

Spring Formation unconformably overlie the folded and faulted

Paleozoic rocks.

The principal structure of the mapped area is the Gass

Peak Thrust, which thrusts Precambrian rocks over Permian rocks, indicating a stratigraphic displacement of 18,000 feet. The Gass Peak Thrust probably passes into a zone of decollement at depth, but its attitude in the subsurface must be inferred from other structures in the area.

The deformation produced by thrust faulting is compli¬ cated by the torsional strain developed as the Gass Peak area was "dragged" northwestward by right-lateral strike-slip faulting oh the adjacent Las Vegas Valley Shear Zone.

Numerous fold axes and faults have strikes that swing west¬ ward in response to this drag. There is also a unique set of faults developed by east-west extension and north-south com¬

L i'VW.* C-vii-O— OA f-'s tv- 1 L pression resulting from the drag. f -,K t i4 T. ' S' J? - c<-\ ;\ A "Vv p t- v-tc-j. V1- ‘ Ar L.:’V'j.' \t~~ /\.t "W tk 49

Little evidence is present in the mapped area for late Tertiary block faulting except isolated terraces of cemented fanglomerate deposits 200 feet above the Recent alluvial fans. 50

CONCLUSIONS

1. Measured sections of the early Paleozoic rock exposed in

the Gass Peak area indicate that there has been consider¬ able thinning in some of the formations, relative to equivalent sections in the Spring Mountains farther west. Notably, the Wood Canyon and Carrara Formations are much thinner than sections of these beds measured by Vincelette (1964) in the Wheeler Pass area of the north¬ west Spring Mountainso Estimated thickness of the Permo-

Carboniferous Bird Spring Formation is intermediate bet¬ ween those measured by Rich (1961) in the Spring Mountains

to the northwest and by Langenheim et al. (1962) in the Arrow Canyon Range to the east.

The total thickness of the Stirling Quartzite in the Gass Peak area is unknown and the Johnnie Formation

is not exposed there.

2. Even though, in much of this area, faulting obscures the stratigraphic relations, and even though the uppermost Permian is probably not present or is covered by the hanging wall of the thrust, the rocks of the Bird Spring Formation exposed east of Gass Peak would make a good section for detailed description and zonation. The rocks are well exposed and very fossiliferous. Results of such

a study would be an interesting comparison to those of Longwell and Dunbar (1936) and Mark Rich (1963), which were done in the Spring Mountains, northwest of the Gass 51

Peak area, and to that of Langenheira et al. (1962), in the Arrow Canyon Range east of the Las Vegas Range.

3. Certain stratigraphic units, like the Dunderberg Shale Member of the Nopah Formation, deserve more widespread recognition as easily identified marker horizons with little lithologic change over a wide area. These would be of great use in studying facies and thickness changes in the adjacent formations.

4. The Wheeler Pass Thrust in the Spring Mountains and the Gass Peak Thrust are very similar because they both fault Stirling Quartzite over Bird Spring Formation for

most of their lengths. This similarity is certainly not

fortuitous and probably indicates that these thrusts were once the same feature. Right-lateral movement on

the Las Vegas Valley Shear Zone divided the original

single fault into two segments whose surface traces have been separated by more than 25 miles. Details of

minor structures associated with the Wheeler Pass Thrust and the Gass Peak Thrust differ considerably, indicating

there may have been some movement on the thrusts subse¬ quent to their separation. Vincelette's (1964) model studies showed that thrusts can develop before strike-

slip faults and independent movement on separated parts of a thrust is possible.

5. One can conclude, from the evidence in the Gass Peak area, that movement on the Las Vegas Valley Shear Zone pro¬

duced drag effects indicating right-lateral strike-slip movement, and that this movement began after initial ! 52. faulting on the Gass Peak Thrust. Minor deformation of the Miocene (?) Horse Spring Formation indicates that some minor movement on the Las Vegas Valley Shear Zone occurred as late as late Miocene time. 53

BIBLIOGRAPHY

Armstrong, R.L., 1963, Geochronology and geology of the eastern Great Basin in Nevada and Utah: unpublished Ph.Do dissertation, Yale University, 202 pages« Ball, S.H., 1907, A geologic reconnaissance in southwestern Nevada and eastern California: U.S. Geol. Survey Bull. 308, p. 1-212.

Barnes, H., and Christiansen, R.L., 1965, in press, Cambrian and Precambrian rocks of the Groom district, Lincoln County, Nevada: U.S. Geol. Survey.

Barnes, H., and Palmer, A.R., 1961, Revision of stratigraphic nomenclature of Cambrian rocks, Nevada Test Site and vicinity, Nevada: in Geol. Survey Research 1961, p. C-100 to C-103. Bissell, H.J., 1962, Pennsylvanian and Permian rocks of Cordilleran area, in Pennsylvanian System in the United States - a symposium: Am. Assoc. Petroleum Geologists, p. 188-263.

Burchfiel, B.C., 1964, Precambrian and Paleozoic stratigraphy of the Specter Range quadrangle, Nye County, Nevada: Am. Assoc. Petroleum Geologists Bull., v. 48, p. 40-56.

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